The overarching goal of the proposed Center for Quantitative Biology remains to instantiate at Princeton a research and teaching environment that fully meets the challenge and opportunity, presented by advances in computation and genomics, to practice a usefully quantitative biological science, sometimes referred to as systems biology. The programs and infrastructure of the Center increase the bandwidth of communication among researchers from different disciplines and departments (including Molecular Biology, Ecology and Evolutionary Biology, Computer Science, Chemistry and Physics). One in four (54/217) papers published with Center support in the last four years is a joint publication between two or more Center faculty. For both undergraduate and graduate students, the Center provides a focus for multidisciplinary teaching and learning; quantitative and biological ideas are integrated from the beginning, with the result that students acquire nearly equal facility in biological and quantitative thinking. The Center's specific aims are: (1) to develop realistic and quantitative models of biological processes; (2) to collect large-scale data sets that comprehensively describe biological processes; (3) to devise new and improved methods for computational analysis and display of complex models, structures and data and to make, upon publication, all underlying data, algorithms and analytical systems publicly accessible; (4) to devise and support new curricula and courses of quantitative biology education for undergraduates, graduate students, and the larger scientific community; and (5) to reduce these ideas to practice in several collaborative and multi-disciplinary projects, each aimed at specific system-level questions in the subject areas of (i) intracellular signaling, (ii) pattern and cell signaling in multicellular organisms, (iii) host-pathogen interactions and (iv) bioinformatics and data visualization. These projects share common quantitative goals, require a common infrastructure (i.e. computation, microarray, imaging and metabolomics core facilities) and all of them benefit from the intellectual synergy and multi-disciplinary cooperation that the Center provides.